On a cloudy summer day in the Richmond district, three members of the Crehan family ride to a park on the sunny side of the city – on one bike.

Mike Crehan uses a cargo bike that has what looks like a wooden wheelbarrow on the front to hold his 3-year-old daughter, Maggie. Attached to the back of the bicycle like a caboose is another bike that 5-year-old Catie rides.

“It’s like a bathtub without water,” Catie explains.

Fed up with cars, traffic, parking and Muni, some parents are using their bicycles as minivans, hauling their children around the hilly streets. They use an array of devices for their rides: child seats attached to the frame, two-wheeled enclosed trailers, extended bikes with room for passengers on the back, trail-a-bikes that let kids ride along behind and "bathtub" bikes.

There is little in-depth research on the safest way to carry young children on bikes, said Randy Swart, director of the Bicycle Helmet Safety Institute, a small nonprofit in Virginia.

“What you get is a lot of anecdotes,” Swart said.

Some parents prefer traditional bike seats to trailers. Lainie Motamedi, president of the board of directors of the San Francisco Bicycle Coalition, said she chose a bike seat to carry her son, Jasper, because of the city's terrain and traffic. The trailers, she said, are heavy when pedaling up hills.

“My other big concern is that (trailers) are low to the ground and pretty far behind you,” Motamedi said. “It was enough just making sure my bike stayed safe, and I didn't want on top of that to deal with cars not realizing that I was towing something behind me.”

Dav Rauch, a father of two who lives in the Mission district, said he prefers a trailer, an enclosed two-wheeled contraption that hitches to the back of the bike.

“It is lower and it’s harder for cars to see, but we do have a big flag,” Rauch said. “When you have a kid on a bike seat, it really throws off the balance and makes it top-heavy, and nothing has as much protection as a trailer.”

The American Academy of Pediatrics recommends the trailer over the bike seat because "a young passenger on an adult's bike makes the bike unstable and increases braking time." Consumer Reports calls the trailer "less risky," because in the event of a collision, a child would fall "three feet from a mounted bike compared with a fall of about 6 inches from a trailer."

Bike crashes in San Francisco are at an all-time high. Last year, there were 630 collisions involving bicycles, up from 599 in 2010, according to a recent report from the San Francisco Municipal Transportation Agency.

But very few involve young bike passengers. Between 2007 and 2011, San Francisco police recorded just one collision involving a child on board a parent’s bike, according to data collected by the California Highway Patrol.

To fend off danger, parents like Crehan say they choose routes with less traffic or separated bike lanes. They say they bike more conservatively when kids are on board.

“I have a good mental map of where the hills are and where the traffic is,” said Crehan, who works as a ferry captain.

There are some basic guidelines for keeping young bike passengers safe. The American Academy of Pediatrics and the U.S. Consumer Product Safety Commission say infants younger than 12 months should not be carried on a bicycle, a recommendation based on the age when most kids can sit upright on their own.

“A year is a convenient benchmark; some kids are probably ready before, some after,” said Swart, of the Bicycle Helmet Safety Institute. “We tell (parents to) take their helmet and their kid to the pediatrician and ask if they're ready to ride on a bicycle.”

Parents can check the label of bike trailers to make sure they meet international safety standards, and they can search the U.S. Consumer Product Safety Commission’s website for products that might pose a danger. In April, Todson recalled 40,000 child bike seats because a hinge was “posing a laceration and fingertip amputation hazard” to its small passengers.

The equipment can be expensive. Trailers can cost hundreds of dollars; the Dutch-style cargo bike that the Crehans own starts at $2,500. And many parents say they don't trust standard brakes on the city's terrain. Crehan found a company in Oregon that builds its cargo bikes with disc brakes. He also installed an electric motor to help get up steep hills.

All of the parents interviewed say they prefer cycling on streets with bike lanes separated from traffic and want the city to create more of them.

“The little plastic cone things really become a wall,” Rauch said. “Those kinds of lanes on very busy streets are very important.”

When they do bike in traffic, some parents say they notice that drivers are more careful around them.

“People are more respectful when the kids are on the bike,” said Dorie Appollonio, a UC San Francisco professor who bikes her kids to school. “And at a minimum, they definitely notice you more – anybody's who's noticing you is someone who's not going to run into you.”

Appollonio, who was not an avid biker, said she started carrying her kids, 2 and 5, after renting bicycles with child seats on a trip to Copenhagen, Denmark.

“It’s easy for kids to spend a lot of time indoors. We’re spending this time outdoors. We’re talking to them. It was nothing like when you’re driving with them,” she said. “You don’t have to worry about parking, you don’t have to worry about getting stuck in traffic.”